Field of the Invention and Related Art Statement
The present invention relates to a power semiconductor device an improved heat dissipating property and also relates to a power semiconductor device having a high breakdown voltage in addition to efficient heat dissipation.
When a large current flows through a large scale semiconductor device such as power thyristor, transistor and diode, a large amount of heat is generated from a semiconductor substrate. In order to dissipate the heat generated from the semiconductor substrate, it has been proposed to arrange electrically conductive members having a large dimension and being made of material having a high heat transmission such as copper on both sides of semiconductor substrate and cooling heat sinks are provided on outer surfaces of the conductive members.
FIG. 1 is a cross sectional view showing a known power semiconductor device, in which on both sides of a semiconductor substrate 20 are arranged electrically conductive members 21 and cooling fins 22 made of copper or aluminum are provided on outer surfaces of the conductive members, respectively. The conductive members are also serve as positive and negative electrodes. The assembly of the semiconductor substrate 20 and conductive members 21 is hermetically sealed by a package 23 made of insulating material such as ceramics. In order to improve the heat dissipating property, the cooling fins 22 have a very large size.
FIG. 2 is a side view illustrating another conventional power semiconductor device comprising the heat dissipating means. In this known power semiconductor device, on both sides of each of semiconductor substrates 20 of semiconductor elements are provided electrically conductive members 21 and each of assemblies of the substrate and conductive members is surrounded by a heat sink 24 made of copper. The semiconductor substrate 20 is hermetically sealed by an insulating package 23 provided between the heat sinks 24. Into each of the heat sinks 24 is inserted one end of a heat pipe 25 and a plurality of cooling fins 26 are provided on the heat pipe.
In the known power semiconductor device shown in FIG. 1, the heat generated by the semiconductor substrate 20 is first transmitted to the conductive members 21 and then is further transmitted to the cooling fins 22, and in the known semiconductor device illustrated in FIG. 2, the heat is transmitted to the heat sink 24 via the conductive members 21 and is transmitted therefrom to the cooling fins 26 by means of the heat pipe 25. It has been experimentally confirmed that there are substantial heat resistances between these components. Furthermore, the heat transmission path from the heat generating point to the heat dissipating point is relatively long. Due to the above mentioned reasons, in the known power semiconductor devices, in order to obtain the adequate heat dissipation, it is necessary to provide the heat sink having a very large surface area, so that the size of the whole device is liable to be large. Even if the large heat sink is used, the cooling ability is limited, and thus the current capacity of the semiconductor device is limited by the heat generation.
Further in order to apply a high voltage to the semiconductor device, the breakdown voltage of the semiconductor substrate has to be improved. This may be achieved by increasing the thickness of the semiconductor substrate. However, an amount of the heat generated from the semiconductor substrate is increased exponentially in accordance with the increase in the thickness of the semiconductor substrate. In this manner, when the thickness of the semiconductor substrate is increased, the heat loss increases abruptly. Therefore, it has been generally practiced to increase the breakdown voltage by connecting a plurality of semiconductor elements in series with each other.
FIG. 2 shows such a construction in which two semiconductor elements, e.g. thyristors are provided between the heat sinks 24 and the heat pipes 25 having the cooling fins 26 are inserted into the heat sinks. When a plurality of semiconductor elements are connected in series with each other while they are clamped between the heat sinks, the size of the whole semiconductor device is increased in proportion to the number of the semiconductor elements, so that the whole construction becomes large and the cost including the assembling and maintenance costs is also increased.